The present invention relates to the reduction iron bearing material, such as iron ore, to metallic iron.
Many different iron ore reduction processes have been described and/or used in the past. The processes may be traditionally classified into direct reduction processes and smelting reduction processes. Generally, direct reduction processes convert iron ores into a solid state metallic form with, for example, use of shaft furnaces (e.g., natural gas-based shaft furnaces), whereas smelting reduction converts iron ores into molten hot metal without the use of blast furnaces.
The conventional reduction processes for production of direct reduced iron (DRI) involve heating beneficiated iron ores to below the melting point of iron, below 1200° C. (2372° F.), either by gas-based processes or coal-based processes. For example, in the gas-based process, direct reduction of iron oxide (e.g., iron ores or iron oxide pellets) employs the use of a reducing gas (e.g., reformed natural gas) to reduce the iron oxide and obtain DRI. Methods of making DRI have employed the use of materials that include carbon such as coal and coke as a reducing agent. A typical composition of DRI is 90 to 95% metallization and 2-4% gangue, but has not been practical for steelmaking processes as a replacement of scrap because its oxygen and gangue content increases energy usage, increase slag volume, and necessitates the addition of costly reagents.
Natural gas-based direct reduced iron accounts for over 90% of the world's production of DRI. Coal-based processes are generally used in producing the remaining DRI production. However, in many geographical regions, the use of coal may be more desirable because coal prices may be more stable than natural gas prices. Further, many geographical regions are far away from steel mills that use the processed product. Therefore, shipment of iron units in the form of iron nuggets produced by a coal-based direct reduction process may be more desirable than use of a smelting reduction process.
Another reduction process in gas-based or coal-based directly reducing iron bearing material to metallic nuggets is often referred to as fusion reduction. Such fusion reduction processes, for example, generally involve the following processing steps: feed preparation, drying, preheating, reduction, fusion/melting, cooling, product discharge, and metallic iron/slag product separation. These processes result in direct reduction of iron bearing material to metallic iron nuggets and slag. Metallic iron nuggets produced by these direct reduction processes are characterized by high grade reduction, nearing 100% metal (e.g., about 96% to about 97% metallic Fe). Percents (%) herein are percents by weight unless otherwise stated.
Unlike conventional direct reduced iron (DRI), these metallic iron nuggets have low oxygen content because they are metallic iron and have little or no porosity. These metallic iron nuggets are also low in gangue because silicon dioxide has been removed as slag. Such metallic iron nuggets are desirable in many circumstances such as use in place of scrap in electric arc furnaces. These metallic iron nuggets can be also produced from beneficiated taconite iron ore, which may contain 30% oxygen and 5% gangue. As a result, with such metallic iron nuggets, there is less weight to transport than with beneficiated taconite pellets and DRI. In addition, generally, such metallic iron nuggets are just as easy to handle as taconite pellets and DRI.
Various types of hearth furnaces have been described and used for direct reduction of metallic iron nuggets. One type of hearth furnace, referred to as a rotary hearth furnace (RHF), has been used as a furnace for coal-based direct reduction. Typically, the rotary hearth furnace has an annular hearth partitioned into a preheating zone, a reduction zone, a fusion zone, and a cooling zone, between the supply location and the discharge location of the furnace. The annular hearth is supported in the furnace to move rotationally. In operation, raw reducible material comprising a mixture of iron ore and reducing material is charged onto the annular hearth and provided to the preheat zone. After preheating, through rotation, the iron ore mixture on the hearth is moved to the reduction zone where the iron ore is reduced in the presence of the reducing material and fused into metallic iron nuggets, using one or more heat sources (e.g., gas burners). The reduced and fused product, after completion of the reduction process, is cooled in the cooling zone on the rotating hearth, preventing oxidation and facilitating discharge from the furnace.
One exemplary metallic iron nugget direct reduction process for producing metallic iron nuggets is referred to as ITmk3® by Kobe Steel. In such a process, dried balls formed using iron ore, coal, and a binder are fed to a rotary hearth furnace. As the temperature increases in the furnace, the iron ore concentrate is reduced and fuses when the temperature reaches between 1450° C. to 1500° C. The resulting products are cooled and then discharged. The intermediate products generally are shell-shaped, pellet-sized metallic iron nuggets with slag inside, from which the metallic iron can be separated.
Another direct reduction process for making metallic iron nuggets has also been reportedly used. See U.S. Pat. No. 6,126,718. In this process, a pulverized anthracite coal layer is spread over a hearth and a regular pattern of dimples is made therein. Then, a layer of a mixture of iron ore and coal is placed over the dimples, and heated to 1500° C. The iron ore is reduced to metallic iron, fused, and collected in the dimples as iron pebbles and slag. Then, the iron pebbles and slag are broken apart and separated.
Both of these direct reduction processes for producing metallic iron nuggets have involved mixing of iron-bearing materials and a carbonaceous reductant (e.g., pulverized coal). Either with or without first forming dried balls, iron ore/carbon mixture is fed to a hearth furnace (e.g., a rotary hearth furnace) and heated to a reported temperature of 1450° C. to approximately 1500° C., to form metallic iron nuggets and slag. Metallic iron and slag can then be separated, for example, with use of mild mechanical action and magnetic separation techniques.
A particular problem with the metallic iron nuggets formed by these previous direct reduction processes was the sulfur content of the nuggets. Sulfur is a major impurity in direct reduced metallic iron nuggets. In the past, carbonaceous reductants utilized in direct reduction processes of iron ore have generally resulted in metallic iron nuggets with at least 0.1% or more by weight sulfur. This high level of sulfur has made the metallic iron nuggets made by direct reduction undesirable in many steelmaking processes, and particularly in the electric arc furnace processes.
Attempts have been made to form metallic iron nuggets with low sulfur content in these previous direct reduction processes using large amounts of additives containing MgCO3 or MgO. Problems, such as increased energy consumption and increased refractory wear, have occurred with fusing these nuggets due to the increases in slag melting temperature caused by MgO in the slag. See EP 1 605 067.